Digital scanners are used to digitize the graphic content, be it color or black and white photographs, artwork, text, and other graphics, from reflective and/or transmissive original documents. This capability is useful in digital document storage, digital content generation, and in more industrial pre-process environments. In this latter application, chemical film-based photographs, for example, can be digitally scanned for pre-print review, followed by production printing.
One of the most common scanner configurations is the flatbed scanner. A transmissive or reflective original document is placed face down on a bed having a glass plate or in a slide tray. A carriage, under the glass plate having a slit aperture facing the document, is then scanned over the original document. A relay optical system in the carriage successively picks-off scan-lines. In the one configuration, a single fold mirror is used in a high-resolution mode, and a series of larger fold mirrors are used in a low-resolution mode. In either case, the fold mirror(s) relay the scanned lines to high/low resolution in the optical imaging system, which images the scan-lines onto a linear or two dimensional image detector, such as a trilinear charge-coupled device (CCD) array or CMOS-based image detector.
Although speed of scanning, for example, is an important capability, one of the major factors in distinguishing scanners is resolution, or the fineness of the detail that the scanner can digitize. Consumer-grade scanners are offered with optical resolutions approaching 600 dots per inch (dpi). Currently, commercial-grade scanners are offered with resolutions of greater than 1,000 dpi, in standard resolution modes. Generally, the increases in resolution are related to the size and levels of integration in the image detectors.
Optical resolution, however, is not increased simply by increasing the density of or number of elements in the image detector since a precision optical imaging system is required to form the image of a scan-line, for example, from the document on the image detector, and its focus must be correct. In the prior art, the scanner's optical imaging system was calibrated in the factory or during a hardware setup procedure associated with a particular document holding device, such as a slide tray for example. A predetermined image was sent through the optical imaging system and received by the image detector. The data from the image detector was then analyzed while adjusting the optical imaging system to obtain the best focus. This best focus position was then stored for all subsequent scanning operations.